A staircase in a residential area has conventionally been constructed as follows: Firstly, a face of inclined land is evenly tamped, foundation concrete with reinforcement is placed on the slope, steps of the staircase are roughly formed of concrete on the foundation, and finally, faces of the roughly-formed steps are mortar finished (FIG. 4).
In the staircase built by the above conventional method, strain is often created between the roughly-formed steps and the face mortar layer, as the rate of expansion and contraction of both materials against temperature change or humidity change differ from each other. This strain causes cracks on the surface of the mortar, through which water can penetrate. Further, a slight amount of air and moisture caught under the mortar layer can cause it to be fallen off the steps because of repeated expansion of air at high temperature and of frozen water at low temperature.
For the above mentioned reasons, the conventional constructing method is not suited to cold regions where land freezes and the time when temperature difference is extremely large.
There is an improved method, compared to the conventional one, for constructing a staircase, in which an L shaped staircase block is used (FIG. 5). The L shaped staircase block is a pre-cast concrete plate block having a cross-sectional side view in which the character L is rotated by 90 degrees counter-clockwise (in a reverse L shaped staircase block, the character L is rotated by 90 degrees clockwise).
In the improved method, because the finishing mortar layer in the conventional method is replaced by the L shaped staircase block, the problem of cracks has been solved. However, a cavity in the mortar filled in the inner space of the L shaped staircase block can be produced due to insufficient filling at the corner of the block or at the joint between blocks. Expansion of air and water in the cavity is apt to cause the problem of peeling between the blocks.
Generally, as a staircase is a means for passing through an inclined land, shortening of the term of works for constructing the staircase is an absolute requirement. Specially, in a repair work for a staircase in use in a residential area, it is strongly required to shorten the term of total traffic suspension. In such a case, it is desirable to utilize the staircase under repair partly, and at the same time, a new staircase is built successively from the opposite end. However, on the basis of the conventional constructing method, it has been difficult to satisfy such requirements, due to various difficulties in the execution of works.
It is publicly known to solve the above-mentioned problems with a pre-cast concrete staircase block as disclosed in the following document (hereinafter referred to as a “prior art”).
Registered utility patent No. 3045821 of Japan
However, in the prior art, such problems have undergone as that the proposed staircase block is too heavy to be handled manually, that the neighbouring blocks shift from the right position, and that a undesirable space which causes separation of the rear surface of the staircase block from the ground concrete due to air layer is likely to occur.